Every day, parents watch the trusted school bus pull away, taking their children to school. There’s no sign on the rear of these buses warning that the exhaust from the tailpipe may be harmful to children’s health. But there should be. The exhaust from diesel fuel—which powers 90 percent of the 454,000 school buses on the road today—has been shown to cause or exacerbate a host of health problems, including asthma, lung disease, and cancer, and has been linked to premature death. Children, who are outdoors for longer periods and are breathing at higher rates than adults, may be particularly vulnerable to its harmful effects (Wiley, 1993). As they wait on the curb, play near idling buses, or even ride safely inside the bus, children may be exposed to this noxious substance every school day.